Technique for Achieving Large-Grain Ag2ZnSn(S,Se)4 Thin Films

ABSTRACT

Techniques for increasing grain size in AZTSSe absorber materials are provided. In one aspect, a method for forming an absorber film on a substrate includes: contacting the substrate with an Ag source, a Zn source, a Sn source, and an S source and/or an Se source under conditions sufficient to form the absorber film on the substrate having a target composition of: AgXZnYSn(S,Se)Z, wherein 1.7&lt;x&lt;2.2, 0.9&lt;y&lt;1.3, and 3.5&lt;z&lt;4.5, and including an amount of the Ag source that is from about 10% to about 30% greater than is needed to achieve the target composition; annealing the absorber film; and removing excess Ag from the absorber film. A solar cell and method for fabrication thereof are also provided.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. application Ser. No. 15/198,795filed on Jun. 30, 2016, the contents of which are incorporated byreference herein.

STATEMENT OF GOVERNMENT RIGHTS

This invention was made with Government support under Contract numberDE-EE0006334 awarded by Department of Energy. The Government has certainrights in this invention.

FIELD OF THE INVENTION

The present invention relates to Ag₂ZnSn(S,Se)₄ (“AZTSSe”) absorbermaterials, and more particularly, to techniques for increasing grainsize in AZTSSe absorber materials by including excess silver (Ag) duringdeposition.

BACKGROUND OF THE INVENTION

The new photovoltaic compound Ag₂ZnSn(S,Se)₄ (“AZTSSe”) has manypromising material properties. AZTSSe has a lower defect density thanthe compound it is based on, Cu₂ZnSn(S,Se)₄ (“CZTSSe”). See, forexample, U.S. patent application Ser. No. 14/936,131 by Gershon et al.,entitled “Photovoltaic Device Based on Ag₂ZnSn(S,Se)₄ Absorber”(hereinafter “U.S. patent application Ser. No. 14/936,131”).

Grain size affects energy conversion efficiency of absorber materials,with a larger grain size generally resulting in a greater efficiency.Therefore, techniques for controlling the grain size in AZTSSe absorbermaterials would be desirable.

SUMMARY OF THE INVENTION

The present invention provides techniques for increasing grain size inAZTSSe absorber materials. In one aspect of the invention, a method forforming an absorber film on a substrate is provided. The methodincludes: contacting the substrate with an Ag source, a Zn source, a Snsource, and at least one of an S source and an Se source underconditions sufficient to form the absorber film on the substrate havinga target composition of Ag_(X)Zn_(Y)Sn(S,Se)_(Z), wherein 1.7<x<2.2,0.9<y<1.3, and 3.5<z<4.5, and including an amount of the Ag source thatis from about 10% to about 30% greater than is needed to achieve thetarget composition; annealing the absorber film; and removing excess Ag,if any, from the absorber film. Optionally, the composition of theabsorber could be measured using Particle Induced X-ray Emission (PIXE)techniques.

In another aspect of the invention, an absorber film is provided that isformed on a substrate by the above method. The absorber film has acomposition of: Ag_(X)Zn_(Y)Sn(S,Se)_(Z), wherein 1.7<x<2.2, 0.9<y<1.3,and 3.5<z<4.5, and wherein the absorber film has an average grain sizeof from about 0.5 micrometers to about 4 micrometers, and rangestherebetween.

In yet another aspect of the invention, a method of forming a solar cellis provided. The method includes: contacting a conducting substrate withan Ag source, a Zn source, a Sn source, and at least one of an S sourceand an Se source under conditions sufficient to form the absorber filmon the conducting substrate having a target composition of:Ag_(X)Zn_(Y)Sn(S,Se)_(Z), wherein 1.7<x<2.2, 0.9<y<1.3, and 3.5<z<4.5,and including an amount of the Ag source that is from about 10% to about30% greater than is needed to achieve the target composition; annealingthe absorber film; removing excess Ag, if any, from the absorber film;forming a buffer layer on the absorber layer; and forming a transparentfront contact on the buffer layer. Optionally, the composition of theabsorber could be measured using Particle Induced X-ray Emission (PIXE)techniques.

In still yet another aspect of the invention, a solar cell is provided.The solar cell includes: a substrate; a conductive layer on thesubstrate; an absorber layer on the conductive layer, the absorber layerhaving a composition of: Ag_(X)Zn_(Y)Sn(S,Se)_(Z), wherein 1.7<x<2.2,0.9<y<1.3, and 3.5<z<4.5, and wherein the absorber layer has an averagegrain size of from about 0.5 micrometers to about 4 micrometers, andranges therebetween; a buffer layer on the absorber layer; and atransparent front contact on the buffer layer.

A more complete understanding of the present invention, as well asfurther features and advantages of the present invention, will beobtained by reference to the following detailed description anddrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram illustrating an exemplary methodology for forming anAZTSSe film on a substrate according to an embodiment of the presentinvention;

FIG. 2 is a cross-sectional diagram illustrating an exemplary startingstructure for fabricating an AZTSSe-based solar cell including asubstrate and a conductive layer on the substrate according to anembodiment of the present invention;

FIG. 3 is a cross-sectional diagram illustrating an AZTSSe absorberhaving been formed on the substrate according to an embodiment of thepresent invention;

FIG. 4 is a cross-sectional diagram illustrating a buffer layer havingbeen formed on the AZTSSe absorber according to an embodiment of thepresent invention;

FIG. 5 is a cross-sectional diagram illustrating a transparent frontcontact having been formed on the buffer layer and metal contacts havingbeen formed on the transparent front contact according to an embodimentof the present invention;

FIG. 6A is an image of an AZTSSe absorber formed using the presenttechniques with a baseline Ag flux, according to an embodiment of thepresent invention;

FIG. 6B is an image of an AZTSSe absorber formed using the presenttechniques with about 20% excess Ag flux, according to an embodiment ofthe present invention;

FIG. 6C is an image of an AZTSSe absorber formed using the presenttechniques with about 40% excess Ag flux according to an embodiment ofthe present invention;

FIG. 7 is a diagram illustrating grain size as a function of Ag sourcetemperature based on the samples shown in FIGS. 6A-C according to anembodiment of the present invention; and

FIG. 8 is an X-ray diffraction of a sample AZTSSe film according to anembodiment of the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Provided herein are techniques for increasing the grain size (i.e.,coarsening) of Ag₂ZnSn(S,Se)₄ (“AZTSSe”) absorber materials. The term“AZTSSe,” as used herein, refers to a material containing silver (Ag),zinc (Zn), tin (Sn), and at least one of sulfur (S) and selenium (Se).AZTSSe materials are described generally in U.S. patent application Ser.No. 14/936,131, the contents of which are incorporated by reference asif fully set forth herein.

As described in U.S. patent application Ser. No. 14/936,131, AZTSSeabsorbers are based on CIGSSe and CZTSSe materials, wherein the Cu orZn, respectively, is swapped out for a different cation (in this caseAg). The term “CIGSSe,” as used herein, refers to a material containingcopper (Cu), indium (In), gallium (Ga), and at least one of S and Se.The term “CZTSSe,” as used herein, refers to a material containing Cu,Zn, Sn, and at least one of S and Se. The use of AZTSSe absorbers avoidsthe Cu—Zn antisite formation and resulting band tailing problemsassociated with conventional materials like CZTSSe. See U.S. patentapplication Ser. No. 14/936,131.

Advantageously, it has been found herein that the grain size of AZTSSeabsorber materials can be controlled by controlling the Ag content ofthe material. Namely, to enhance the performance of the resultingdevices, an excess amount of Ag (i.e., an amount of Ag in excess of whatis needed to produce the desired final composition) can be used duringformation of the material to increase (i.e., coarsen) the grain size ofthe AZTSSe material.

As will be described in detail below, according to an exemplaryembodiment, the final AZTSSe material has a formula:

Ag_(X)Zn_(Y)Sn(S,Se)_(Z),

wherein 1.7<x<2.2, 0.9<y<1.3, and 3.5<z<4.5, and an excess amount of Ag(in excess of what is needed to achieve this formula) of from about 10%to about 30%, and ranges therebetween, is added during deposition of thematerial. Thus, to use a simple example to illustrate this concept, ifthe final desired composition is Ag₂ZnSn(S,Se)₄, then Ag_(2.2) would beadded to get 10% extra Ag during deposition of the material.Proton-Induced X-ray Emission (PIXE) is an analytical technique that canbe used to measure the stoichiometry of the elements making up theabsorber, and is sensitive enough to measure the excess Ag from thenominal level.

Without being bound by any particular theory, it is thought that theexcess Ag acts as flux (i.e., flow) agent during the AZTSSe formationprocess. Namely, grain size is controlled by the movement of atoms intoa grain from adjacent grains. Grain boundaries act as barriers to thisatom diffusion. By adding a flux agent, the atoms can flux through tothe other grains. Specifically, during annealing (see below), liquid (orhigh-diffusivity) Ag₂Se or Ag₈SnSe₆ (secondary phases formed as a resultof the excess Ag) increases kinetics of atomic transport across grainboundaries.

An exemplary process for forming an AZTSSe film on a substrate is nowdescribed by way of reference to methodology 100 of FIG. 1. In order toconfigure the deposition process, a target composition of the AZTSSematerial is chosen in step 102. This will enable one to know how muchextra Ag to add (as a flux agent) during the deposition. According to anexemplary embodiment, the target composition isAg_(X)Zn_(Y)Sn(S,Se)_(Z), wherein 1.7<x<2.2, 0.9<y<1.3, and 3.5<z<4.5.

In step 104, AZTSSe is deposited onto the substrate, wherein an amountof Ag used during the deposition is from about 10% to about 30%, andranges therebetween, greater than what is needed for the targetcomposition. For instance, as per the example provided above, if thetarget composition is Ag₂ZnSn(S,Se)₄, then increasing the Ag flux duringthe deposition so as to achieve a composition of Ag_(2.2)ZnSn(S,Se)₄ inthe final film would constitute an excess amount of Ag of 10%.

According to an exemplary embodiment, the AZTSSe is deposited onto thesubstrate using a thermal evaporation process. By way of example only,the substrate is heated within a vacuum chamber, and held at atemperature of from about 100 degrees Celsius (° C.) to about 400° C.,and ranges therebetween. Thermal evaporation is then used toco-evaporate Ag, Zn, and Sn, and S and/or Se from their respectivesources onto the substrate. In general, a thermal evaporation processinvolves heating and evaporation of a target source material (in thiscase Ag, Zn, Sn, S and/or Se evaporation source materials) with aheating element (e.g., as in a Knudsen cell or a thermal cracking cell)to transform constituent atoms into the gaseous phase (forming a sourcegas). When the source gases meet the substrate, these atoms can reactand precipitate into a film which deposits on the substrate, forming alayer of the compound AZTSSe material on the substrate.

The temperature at which the source materials are heated can becontrolled to regulate the amount of the corresponding material in thesource gas. Namely, the present techniques involve using an excessamount of Ag during deposition. That excess amount of Ag can begenerated by controlling the temperature of the Ag source to achieve ahigher Ag flux. See also FIGS. 6A-C, described below.

According to an exemplary embodiment, a thermal cracking source is usedfor the S and the Se, wherein the S and the Se sources have two zones.One zone heats the bulk source material to make it evaporate, andanother zone cracks the material, e.g., from S8 to elemental S (i.e.,from a long-chain molecule comprising up to 8 chalcogen atoms to smallermolecules, or in the limit one atom of S or Se). The flux of the Sand/or Se can be controlled both by regulating the bulk zone temperatureas well as by controllably opening of a needle valve between the twozones. This thermal cracking for the S and Se sources can be carried outusing a commercially available thermal cracking cell. This process canbe used to control the ratio of S to Se in the material (i.e., theratios of S/(S+Se) or Se/(S+Se)). By regulating the ratio of S to Se inthe material, one can control the band gap of the AZTSSe material. Byway of example only, a suitable apparatus that may be employed inaccordance with the present techniques to supply controlled amounts of Sand/or Se during kesterite absorber film growth is described, forexample, in U.S. Patent Application Publication Number 20120100663 byBojarczuk et al., entitled “Fabrication of CuZnSn(S,Se) Thin Film SolarCell with Valve Controlled S and Se” (hereinafter “U.S. PatentApplication Publication Number 2012/0100663”), the contents of which areincorporated by reference as if fully set forth herein. U.S. PatentApplication Publication Number 2012/0100663 describes an apparatus thatincludes two separate cracking cells, one for S and another for Se. Eachcracking cell can be independently regulated to control the amounts of Sand Se present during deposition. Thus, according to an exemplaryembodiment, the S and Se source gas in the vapor chamber is introducedvia one or more independently controllable cracking cells.

Accordingly, (gaseous) sources of Ag, Zn, Sn (e.g., by coevaporation),and at least one of S and Se (e.g., by cracking) can be created in thevacuum chamber. When contacted with the heated substrate, atoms fromthese sources can react and precipitate on the substrate forming anAZTSSe film. Suitable conditions for this AZTSSe deposition processinclude, but are not limited to, a temperature of from about 10° C. toabout 450° C., and ranges therebetween, a duration of from about 1minute to about 200 minutes, and ranges therebetween, and a vacuumchamber pressure of from about 1×10⁻⁵ Torr to about 5×10⁻¹⁰ Torr, andranges therebetween.

Following deposition, the AZTSSe film is annealed in step 106. Annealingimproves the crystal grain structure as well as the defect structure,and in some cases may be necessary to form a material having a kesteritestructure. According to an exemplary embodiment, conditions for theanneal performed in step 106 include a temperature of from about 430° C.to about 550° C., and ranges therebetween, for a duration of from about20 seconds to about 10 minutes, and ranges therebetween.

Optionally, the anneal can be carried out in an environment containingexcess chalcogen, e.g., excess S and/or Se. See, for example, U.S. Pat.No. 8,642,884 issued to Mitzi et al., entitled “Heat Treatment Processand Photovoltaic Device Based on Said Process” (hereinafter “U.S. Pat.No. 8,642,884”), the contents of which are incorporated by reference asif fully set forth herein. U.S. Pat. No. 8,642,884 describes use of asulfurization or selenization heat treatment process to passivate thelayers and interfaces in the device.

As provided above, it is proposed that the excess Ag acts as a fluxagent during the deposition process, enabling a greater atomic mobilityacross the grain boundaries and thereby enhancing grain size. Forinstance, according to an exemplary embodiment, following the anneal theresulting AZTSSe film has an average grain size of from about 0.5micrometers (μm) to about 4 μm, and ranges therebetween depending on theexcess Ag provided during the annealing stage. By way of example only,grain size can be measured as the average cross-sectional dimension ofthe grain.

Excess Ag used during deposition can result in the formation ofsecondary phases (i.e., portions of the deposited material that are notAZTSSe). One can observe whether or not these phases are present usingX-ray diffraction (see FIG. 8, described below). When present, thesesecondary phases can be removed selectively from the grain boundaries.By way of example only, secondary phases can include Ag₂Se and Ag₆SnSe₈.The goal is to produce a single-phase AZTSSe material. Therefore, instep 108 the excess Ag (i.e., these secondary phases) are removed,leaving behind single-phase AZTSSe in the film.

According to an exemplary embodiment, an etch is used remove the excessAg. For instance, potassium cyanide (KCN) or sodium cyanide (NaCN) canbe used as etchants to remove Ag₆SnSe₈ and Ag₂Se. Thus, according to anexemplary embodiment, the etch is performed in KCN and/or NaCN.

As a result of the above-described process, a film of large-grainedsingle-phase AZTSSe is produced on the substrate. The AZTSSe film canhave a variety of different applications. By way of example only, theAZTSSe film can be used as the absorber material in a photovoltaicdevice. Accordingly, an exemplary embodiment employing the presenttechniques to form an AZTSSe absorber-based solar cell is now describedby way of reference to FIGS. 2-5.

As with the embodiment described above, the process begins with asuitable substrate 202 on which the AZTSSe absorber will be formed. Forsolar cell applications, if not conductive itself, the substrate 202 iscoated with a conductive layer 204 which will serve as an electrode ofthe solar cell. In one exemplary configuration, the solar cell will beconstructed as a series of layers as a stack on the substrate 202, andanother electrode will be formed on top of the stack. In that case, theconductive layer 204 might also be referred to herein as a bottomcontact/electrode, and the other the top/front contact/electrode.

According to an exemplary embodiment, the substrate 202 is glass,ceramic, metal foil, or plastic substrate. Suitable materials for theconductive layer 204 include, but are not limited to, metal-containingmaterials (such as molybdenum (Mo) and/or transparent conducting oxides(TCOs) such as fluorinated tin oxide (SnO₂:F, FTO), tin-doped indiumoxide (In₂O₃:Sn, ITO), and doped ZnO such as aluminum-doped ZnO (ZnO:Al,AZO). The conductive layer 204 can be formed on the substrate 202 usinga process such as evaporation, sputtering, atomic layer deposition(ALD), or spray pyrolysis. By way of example only, the conductive layer204 is formed having a thickness of from about 0.1 μm to about 4 μm, andranges therebetween.

Next, as shown in FIG. 3, an AZTSSe absorber 302 is formed on thesubstrate 202 (or on the conductive layer 204-coated substrate 202).According to an exemplary embodiment, the AZTSSe absorber 302 is formedaccording to methodology 100 of FIG. 1, described above. Namely, basedon a selected target composition of the AZTSSe absorber 302, an excessamount of Ag (e.g., from about 10% to about 30%, and rangestherebetween) is used during the deposition which, as described above,serves to enhance the grain size of the AZTSSe. After deposition, theAZTSSe is annealed, and excess Ag (e.g., secondary phases such as Ag₂Seand Ag₆SnSe₈) is then removed. The result is a single-phase AZTSSeabsorber 302 having been formed on the substrate 202 (or on theconductive layer 204-coated substrate 202).

As described in detail above, the excess Ag during deposition of theAZTSSe acts as a flux agent that increases kinetics of atomic transportacross grain boundaries via the high-diffusivity secondary phases (e.g.,Ag₂Se or Ag₈SnSe₆) that are formed during annealing. Thus, a coarsegrain can be achieved. For instance, according to an exemplaryembodiment, the AZTSSe absorber 302 has an average grain size of fromabout 0.5 μm to about 4 μm, and ranges therebetween.

A buffer layer 402 is then formed on the AZTSSe absorber 302. See FIG.4. It is notable that the features in the figures may not be drawn toscale, e.g., in practice the absorber is much thicker than the buffer.As described in U.S. patent application Ser. No. 14/936,131, AZTSSe isinherently an n-type material. Thus, traditional buffer materials, suchas cadmium sulfide (CdS) which is also an n-type material, might notsuffice. On the other hand, p-type buffer layers would be suitable foran AZTSSe-based solar cell. By way of example only, viable buffermaterials for an AZTSSe-based solar cell include, but are not limitedto, copper(I) oxide (Cu₂O). nickel(II) oxide (NiO), zinc telluride(ZnTe), aluminum phosphide (AlP), molybdenum trioxide (MoO₃), cadmiumtelluride (CdTe), copper(I) iodide (CuI), molybdenum(IV) oxide (MoO₂).molybdenum disulfide (MoS₂), molybdenum diselenide (MoSe₂), andcombinations thereof, where the roman numerals in the parenthesesindicate the valence state of the metal atoms in the compound.Alternatively, as described in U.S. patent application Ser. No.14/936,131 (see, e.g., FIG. 7), doping can be employed to modify thecarrier concentration and make the AZTSSe absorber compatible withconventional device materials like CdS. For instance, incorporatingindium (In) into the AZTSSe absorber will make the absorber less n-type.

A transparent front contact 502 is next formed on the buffer layer 502.Suitable materials for forming the transparent front contact 502include, but are not limited to, a transparent conductive oxide (TCO)such as indium-tin-oxide (ITO) and/or aluminum (Al)-doped zinc oxide(ZnO) (AZO), which can be deposited onto the buffer layer 502 using aprocess such as sputtering. As shown in FIG. 5, metal contacts 504 mayalso be formed on the transparent front contact 502. Suitable materialsfor forming the metal contacts 504 include, but are not limited to,aluminum (Al) and/or nickel (Ni), which can be deposited onto thetransparent front contact 504 using a process such as thermal orelectron-beam (e-beam) evaporation.

As provided above, the amount of the components in the AZTSSe absorbercan be controlled by the temperature of the respective source. Inparticular, in order to generate the excess amount of Ag duringdeposition, one can simply control the temperature at which the Agsource is heated to control how much Ag is present in the source gasduring evaporation. FIGS. 6A-C depict images of AZTSSe absorber samplesprepared at three different Ag source temperatures, corresponding tobaseline (FIG. 6A), 20% (FIG. 6B) and 40% excess Ag flux (FIG. 6C).

Specifically, FIG. 6A is an image 600A of an AZTSSe absorber formedusing the present techniques with an Ag source temperature of 907degrees Celsius (° C.), corresponding to a baseline Ag flux. This samplein image 600A had a grain size (average±Std. Dev.) of 1.15±1.05 μm. FIG.6B is an image 600B of an AZTSSe absorber formed using the presenttechniques with an Ag source temperature of 917° C. By increasing thetemperature by 10 degrees (i.e., as compared to the sample shown inimage 600A), about 20% more Ag was generated in the source gas. Thissample in image 600B had an (increased) grain size (average±Std. Dev.)of 1.47±1.28 μm. FIG. 6C is an image 600C of an AZTSSe absorber formedusing the present techniques with an Ag source temperature of 922° C. Byincreasing the temperature by 15 degrees (i.e., as compared to thesample shown in image 600A), about 40% more Ag was generated in thesource gas. This sample in image 600C had an (increased) grain size(average±Std. Dev.) of 2.15±2.00 μm.

The results shown in FIGS. 6A-C are represented graphically in FIG. 7.In FIG. 7, the Ag source temperature (measured in ° C.) is plotted onthe x-axis, and the grain size (measured in μm) is plotted on they-axis. As shown in FIG. 7, the increase in Ag source temperaturecorrelates with an increase in grain size due, as described above, tothe excess Ag present during deposition.

FIG. 8 is an X-ray diffraction plot of a sample AZTSSe. As providedabove, X-ray diffraction can be used to observe whether or not Agsecondary phases are present in the sample. When present, the secondaryphases can be removed from the grain boundaries.

Although illustrative embodiments of the present invention have beendescribed herein, it is to be understood that the invention is notlimited to those precise embodiments, and that various other changes andmodifications may be made by one skilled in the art without departingfrom the scope of the invention.

What is claimed is:
 1. An absorber film formed on a substrate by: i)contacting the substrate with an Ag source, a Zn source, a Sn source,and at least one of an S source and an Se source under conditionssufficient to form the absorber film on the substrate having acomposition of Ag_(X)Zn_(Y)Sn(S,Se)_(Z), wherein 1.7<x<2.2, 0.9<y<1.3,and 3.5<z<4.5, and including an amount of the Ag source that is fromabout 10% to about 30% greater than is needed to achieve thecomposition, ii) annealing the absorber film, and iii) removing excessAg, if any, from the absorber film, wherein the absorber film has anaverage grain size of from about 0.5 micrometers to about 4 micrometers,and ranges therebetween.
 2. The absorber film of claim 1, wherein theexcess Ag forms a secondary phase selected from the group consisting of:Ag₂Se, Ag₆SnSe₈, and combinations thereof.
 3. The absorber film of claim1, wherein the excess Ag is removed using an etchant selected from thegroup consisting of: potassium cyanide, sodium cyanide, and combinationsthereof.
 4. The absorber film of claim 1, wherein the substrate is aglass, ceramic, metal foil, or plastic substrate.
 5. The absorber filmof claim 1, further comprising a conductive layer on the substrate. 6.The absorber film of claim 5, wherein the conductive layer comprises ametal-containing material.
 7. The absorber film of claim 6, wherein themetal-containing material comprises molybdenum.
 8. The absorber film ofclaim 5, wherein the conductive layer comprises a transparent conductingoxide.
 9. The absorber film of claim 8, wherein the transparentconducting oxide is selected from the group consisting of: fluorinatedtin oxide, tin-doped indium oxide, doped zinc oxide, aluminum-doped zincoxide.
 10. The absorber film of claim 5, wherein the conductive layerhas a thickness of from about 0.1 μm to about 4 μm, and rangestherebetween.
 11. A solar cell, comprising: a substrate; a conductivelayer on the substrate; an absorber layer on the conductive layer, theabsorber layer having a composition of: Ag_(X)Zn_(Y)Sn(S,Se)_(Z),wherein 1.7<x<2.2, 0.9<y<1.3, and 3.5<z<4.5, and wherein the absorberlayer has an average grain size of from about 0.5 micrometers to about 4micrometers, and ranges therebetween; a buffer layer on the absorberlayer; and a transparent front contact on the buffer layer.
 12. Thesolar cell of claim 11, wherein the buffer layer comprises a materialselected from the group consisting of: copper(I) oxide, nickel(II)oxide, zinc telluride, aluminum phosphide, molybdenum trioxide, cadmiumtelluride, copper(I) iodide, molybdenum(IV) oxide, molybdenum disulfide,molybdenum diselenide, and combinations thereof.
 13. The solar cell ofclaim 11, further comprising: metal contacts on the transparent frontcontact.
 14. The solar cell of claim 11, wherein the metal contactscomprise a material selected from the group consisting of: aluminum,nickel and combinations thereof.
 15. The solar cell of claim 11, whereinthe conductive layer comprises a metal-containing material.
 16. Thesolar cell of claim 15, wherein the metal-containing material comprisesmolybdenum.
 17. The solar cell of claim 11, wherein the conductive layercomprises a transparent conducting oxide.
 18. The solar cell of claim17, wherein the transparent conducting oxide is selected from the groupconsisting of: fluorinated tin oxide, tin-doped indium oxide, doped zincoxide, aluminum-doped zinc oxide.
 19. The solar cell of claim 11,wherein the conductive layer has a thickness of from about 0.1 vim toabout 4 μm, and ranges therebetween.
 20. The solar cell of claim 11,wherein the transparent front contact comprises a transparent conductiveoxide selected from the group consisting of indium-tin-oxide,aluminum-doped zinc oxide and combinations thereof.